The coking oven is of the type which includes heating flues cooperating in pairs, high level and low level combustion stages and a flue gas recirculation (a recirculating path) at the bottom of heating flues.
It is known that nitrous oxide produced in coking ovens is primarily the so-called thermal NO.sub.x whose production rate depends almost linearly on the product of oxygen and nitrogen concentrations in the flame, and exponentially on the flame temperature.
Known measures for reducing NO.sub.x generation are concerned with the reduction of the flame temperature by recirculating the flue gas, with the reduction of oxygen and nitrogen concentrations through partial combustion.
The principle of the flue gas recirculation in coking ovens is effected in the Koppers recirculation type ovens. In the latter ovens, flue gas is admixed through one or two openings in each second partition or header wall at the bottom level of the heating flue, with the air- and heating gas current. This measure markedly reduces the NO.sub.x production rate primarily due to the reduction of the maximum flame temperature but also due to the reduction of the O.sub.2 and N.sub.2 concentrations.
The principle of partial combustion is employed in coking furnaces operating with a stage heating.
In the endeavour to further lower the NO.sub.x emission in coking furnaces, theoretical and experimental investigations have been made in this direction. As a result of these studies it has been found that a combination of the principle of flue gas recirculation (a recirculation current heating) with the combustion in two stages (stagewise heating) can achieve a further reduction of the produced NO.sub.x.
In principle, the combination of the stagewise heating with the recirculating current heating in coking ovens is known. The abovementioned investigation however has shown that an arbitrary combination of the stage heating with the recirculating current heating does not lead automatically to a noticeable reduction of NO.sub.x.